Lately, a working current in a motor driving device has increased in accordance with an enhancement in performance and function of electronic devices used in the motor driving device, thereby requiring semiconductors to be used to meet a high current. A motor driving device used conventionally is shown in FIG. 16. An example of this conventional motor driving device will be described below with reference to FIG. 16.
In FIG. 16, reference numerals respectively indicate: 1a an IGBT; 1b a diode; 3 a high temperature solder; 4 a metallic element; 5 a sealing resin; 6 a solder; 7 a circuit board; 10 a heat radiation plate; 10a a projecting screw bed; 11 a screw; 12 a surface mounting electronic component (passive element); 13 a surface mounting electronic component (semiconductor element); 14 a metallic base circuit board; 15 a silicone grease; 18 a metallic wire; 19 a metallic lead; 22 a projecting connector; 23 a dented connector; and 24 an insulation resin.
A method for manufacturing this thus-constituted conventional motor driving device will be discussed hereinbelow.
Semiconductor components such as the IGBT 1a, the diode 1b and the like are connected by the high temperature solder 3 to metallic elements 4. The IGBT 1a and the diode 1b, and these semiconductor components and metallic lead 19, are electrically joined together with use of the metallic wire 18. An aluminum wire or a gold wire is normally used as the metallic wire 18. In using a metallic wire 18 formed of, e.g., aluminum, the metallic wire 18 is connected to a second electrode of one of these semiconductor components by performing wedge bonding of the aluminum wire. The second electrode is present opposite to a first electrode of this semiconductor component, which is joined to the metallic element 4. The second electrode of the semiconductor component is formed of aluminum. Oxide films of aluminum on surfaces of both the second electrode and the metallic wire 18 are removed when the second electrode and the metallic wire are pressed into contact with each other with an ultrasonic energy being applied thereto in an ordinary temperature state. The second electrode and the metallic wire 18 are thus joined. The metallic wire 18 joined to the second electrode of the one semiconductor component is routed to the metallic lead 19 obtained by plating copper with tin, and is joined to the metallic lead 19 by performing a wedge bonding method.
Thereafter, for a purpose of physically protecting the one semiconductor component and the metallic wire 18, and improving reliability, the semiconductor component 1 and the metallic wire 18 are coated and sealed with the sealing resin 5 by performing a transfer molding technique or injection molding technique. The metallic lead 19 is bent and cut by molds to be even with the metallic element 4. During this sequence of procedures, an electronic component named “TO-220” comprising the one semiconductor component, the high temperature solder 3, the metallic element 4, the metallic wire 18, the metallic lead 19 and the sealing resin 5 is completed.
After a solder paste is printed onto the metallic base circuit board 14, various components such as the above electronic component “TO-220”, the projecting connector 22, and the like are placed on the metallic base circuit board. The metallic base circuit board 14 in its entirety is put into a heating furnace, whereby the solder paste is melted. The solder paste is then set by being returned to an ordinary temperature. This set solder 6 electrically and physically joins the metallic base circuit board 14 to various electronic components such as the electronic component “TO-220”, the projecting connector 22, and the like.
For providing electrical insulation, the sealing resin 24 is applied to the metallic base circuit board 14 in its entirety. The metallic base circuit board 14, with the various electronic components, is put into a reduced pressure furnace to remove bubbles mixing inside the sealing resin 24, and then put into a heating furnace to set the sealing resin 24.
Next, the silicone grease 15 is applied to the heat radiation plate 10. The metallic base circuit board 14 is brought into intimate contact with the heat radiation plate 10 and fixed by screws. Then, by inserting the projecting connector 22 into the dented connector 23 after registering the projecting connector 22 mounted on the metallic base circuit board 14 with the dented connector 23 mounted on the circuit board 7, the circuit board 7 is brought into intimate contact with the projecting screw bed 10a and fixed by screws 11.
In the manner as above, the process of mounting to the metallic base circuit board 14 electronic components which include electronic components “TO-220” for switching a motor driving current and requiring heat radiation, and the process of combining the circuit board 7 including the circuit for controlling the electronic components “TO-220” and requiring no heat radiation, are completed.
The above-described arrangement generates a loss caused by a resistance of metallic wires 18 and metallic leads 19 and also a stray inductance because of a length of the wires 18 and leads 19. In addition, for example, since the electronic component “TO-220” is equipped with a metallic lead 19, a larger area than an area of the electronic component “TO-220” is required for the metallic base circuit board 14, thereby impeding miniaturization and high-density mounting.
Meanwhile, a motor driving device for electric products alike has been required to be made compact and highly efficient in terms of heat radiation to meet a recent trend towards lighter, thinner, shorter and smaller construction of electric products. However, when bubbles are present inside the high temperature solder 3, the bubbles obstruct heat transfer generated by a semiconductor component, thereby increasing a resistance from the semiconductor component to the metallic element 4. As a result, only a bubble part becomes high in temperature, which leads to breakage of the semiconductor component 1 in a worst case.
As described hereinabove, the metallic wire 18 is joined to the second electrodes of semiconductor components by performing a wedge bonding method with the aluminum wire. In the conventional art, the metallic wire 18 is limited in thickness due to this joining method, and at the same time the metallic wire 18 is limited in length due to an arrangement of substrate electrodes, thereby making it impossible to reduce a wiring resistance. Coping with an on-state resistance decrease in consequence of recent progress of semiconductor components is thus hindered, with an imposing problem of noise increase resulting from electrical signals' requirement of a high frequency and a large current.